Squintz said:instead of an alarm how about some sort of shocker device. I guess it could be considered and alarm. Make the handles rubber mostly but on each handle add a strip of conductive metal which could be tied to a miniature alarm style circuit mounted in a hard to reach place such as the goose neck of the bike. Most people mount the bike by grabing both handles and swinging their leg over. So if one handle is Negative and the other is positive with some nice juice running through it you could shock the hell out of someone hopefully not killing them and im sure they will think twice about trying to ride the bike again.
Then if you really like you bike put a lowjack system on it